In many uses of refrigerant compressors, it is desirable to be able to reduce the capacity or volume of displacement of the compressor under certain operating conditions. Generally, the compressor motor is designed for a maximum peak load and operation of the compressor above design loads can cause motor and/or bearing damage. In many instances compressors are equipped with motor protection devices that are designed to terminate compressor motor operation when abnormal operating conditions are encountered. In most instances, refrigeration system compressors employed in cooling enclosures are designed to function efficiently within certain ambient outdoor temperatures. The peak load increases as the condenser temperature rises as in the case when ambient outdoor temperatures are above the design range, in which case the condenser which is located in the outdoor ambient loses its ability to dispel heat and, accordingly, a high lead pressure exists in the refrigerant discharged thereto. It should be noted that each time motor operation is terminated, the air conditioning process stops including dehumidification. When termination of the system is affected by a motor protection device, the system may be off for several minutes and may then make several attempts to start before continuous operation begins. This cycling of the compressor results in the temperature and humidity level of the air being conditioned to be out of the selected comfort level.
If the compressor continues to operate at above design peak load, the compressor bearing damage may occur, however, it would be advantageous to keep the compressor operating during those periods of extreme outdoor temperatures even at less than normal efficiency which, in many instances, will maintain a desired comfort level by providing some degree of cooling and dehumidification of indoor air.